The present invention relates to a method of forming optical waveguide fibers having improved core roundness and core-clad axis concentricity and more particularly, to a method of treating the preform bait tubes prior to forming deposits on the inner surfaces thereof.
Optical waveguides, which are the most promising medium for use in optical communication systems operating in the visible or near visible spectrum, normally consist of an optical filament having a transparent core surrounded by a transparent cladding material having a refractive index lower than that of the core material.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in filamentary form. Certain glass-making processes, particularly vapor deposition processes have been commonly employed in the formation of optical waveguide blanks. In one such process, one or more layers of glass are formed on the inner surface of a glass bait tube by chemical vapor deposition or by other known techniques. Ordinarily, the coated bait tube has at least two compositional regions. The interior region will ultimately form the core of the resultant optical fiber, and the exterior region will form the cladding thereof. The remaining critical step involves drawing the relatively large diameter cylindrical preform into a relatively small diameter fiber. Prior to drawing the preform into a fiber, the preform is usually collapsed into a smaller diameter preform or preferably into a solid cylindrical mass.
Nonuniformities in the bait tube can adversely affect the resultant optical waveguide fiber. For example, a non-circular bait tube will generally result in a fiber which has a non-circular core, even though the outer surface of the fiber is circular in cross-section. Furthermore, it appears that softer core materials are more susceptible of being forced by the collapsing bait tube into cores which are out-of-round. Nonuniform bait tube wall thickness at a given cross-section thereof can result in an optical waveguide fiber in which the axis of the core and that of the cladding are not concentric. Also, a bait tube, the outer surface of which is very round, can become out of round during the step of depositing the core material therein, and/or the deposited core material can be of nonuniform thickness if the bait tube has poor wall thickness uniformity.
Fibers having out-of-round cores and fibers wherein the core is not concentric with the outer cladding surface incur inordinately high splice losses when coupled together. These detrimental features of optical waveguides cores must be minimized if such fibers are to be successfully utilized in long distance waveguide transmission systems.
Some of the terms used in the following description to denote conditions of the bait tube or fiber are defined as follows. The degree of bait tube non-circularity, herein referred to as "percent out of round" (%OOR), is defined as [(max OD/min OD)-1].times.100, wherein OD is the tube outer diameter. Core out-of-round, referred to herein as COOR, is defined as min d/max d, where d is the fiber core diameter. The term "% siding" which is used herein to describe non-uniformity of bait tube wall thickness, is defined as [(max t/min t)-1].times.100, where t is the wall thickness. Preferred bait tubes possess low values of %OOR and % siding, and preferred optical waveguide fibers have a value of COOR as near as possible to 1.0. For each of the bait tube and fiber parameters defined herein, measurements are made at a given cross-section of the tube or waveguide.
One standard for fiber COOR was determined by correlating the effect of out-of-round cores on splicing loss. The results are shown in Table 1.
TABLE 1 ______________________________________ Type Fiber COOR Range Splicing Loss (dB) ______________________________________ Class 1 0.98-1.00 0.23 Class 2 0.95-0.97 0.24 Class 3 0.90-0.94 0.43 ______________________________________
According to this study, there was no significant difference between splice loss of class 1 and class 2 fibers; however, there was a significant difference between class 2 and class 3 fibers. Based on this study, it is desirable to employ in the process of manufacturing optical waveguide fibers, only bait tubes which are capable of providing fibers having COOR values of about 0.95 or greater.
A commonly employed, commercially available bait tube, hereinafter referred to as a "standard borosilicate tube" is drawn on a "Vello" machine described in Glass: The Miracle Worker by C. J. Phillips, (1941) Pitman Publishing Company, pp. 209-212. The tube, which comprises about 96 wt.% SiO.sub.2 and about 4 wt.% B.sub.2 O.sub.3, is formed of glass manufactued in accordance with the teachings of U.S. Pat. Nos. 2,106,744 and 2,221,709. A study of a random sampling of such standard borosilicate tubes revealed that they possessed a mean % OOR of about 1.2% and a mean % siding of about 10%. If such tubes were to be employed without modification in the manufacture of optical waveguide fibers by depositing by chemical vapor deposition a layer of core material on the inner surface thereof, the mean COOR of the resultant fibers would be lower than the minimum acceptable level of COOR that has been established for long distance telecommunications purposes. Indeed, a random sampling of fibers drawn from such standard borosilicate tubes revealed that the average COOR value was less than 0.90.
In order to improve the COOR values of fibers produced from standard borosilicate tubes, such tubes have been shrunk on a precision mandrel, or they have been shrunk on a mandrel and ground to improve the percent siding thereof. Such additional treatment of the standard borosilicate bait tube increases the cost thereof up to about 20 times the cost of the tube as drawn. Also, the mandrel can contaminate the inner surface of the bait tube. It would be advantageous to employ the standard borosilicate bait tube because of the low cost thereof and yet to increase the yield of fibers having acceptable levels of COOR.
U.S. Pat. No. 4,154,591 issued to W. G. French et al. teaches a method whereby expansion of the hollow bait tube prior to the deposition step improves the circularity of an otherwise distorted bait tube. The diameter of the bait tube is increased during the first traversal of the heat source by applying additional positive pressure within that tube, and subsequently the process of forming glass layers on the inner surface of the tube proceeds as heretofore described. Although tube circularity is improved, the % siding is adversely affected by this method. If the % siding of the as-drawn bait tube is extremely low, the expanded bait tube may be satisfactory for use in the vapor deposition process which forms the waveguide core layer. However, when the aforementioned standard borosilicate tubes are expanded in accordance with the teachings of the French et al. patent, the % siding increases, i.e., the wall thickness nonuniformity becomes worse. For example, when five randomly selected standard borosilicate tubes having an OD of about 25 mm, and a wall thickness of about 2.3 mm and having an average % siding of 6.28 were inflated to an OD of about 32 mm, the % siding increased by about 5% to an after inflation value of about 11%. When tube roundness is improved by expanding the tube OD to a value greater than the original tube OD, the % siding will increase, i.e., the tube wall thickness uniformity will be deleteriously affected. During subsequent processing, a tube having a high % siding will more readily become out-of-round. Also, it is thought that nonuniform wall thickness can cause the thickness of the deposited core material to be nonuniform.
The aforementioned French et al. patent also describes a method of collapsing a coated preform pressure to obtain a solid preform having a more symmetrical and circular cross-section. Although such a pressure-collapse technique is advantageous in that it prevents further distortion of a preform during the preform collapse step, it cannot ensure the formation of a fiber having a round core if the bait tube is initially out of round or if it becomes out of round during deposition of the core material therein.